Question

Why do we need a blank sample for kjeldahl method?

Answer 1

The determination of the nitrogen content of proteins and amino acids
is of fundamental importance for many investigations, inasmuch as the
validity of other phases of the study often rests upon the accuracy of this
determination. The Kjeldahl method for nitrogen has undergone many
modifications since its inception, usually with the object of decreasing the
period required for digestion. In the last 15 years, the wide-spread use of
micro modifications of this method has introduced further variations in
the procedure. The Kjeldahl method has had such universal acceptance
that, in many publications, no details as to the accuracy of the particular
procedure, the oxidizing and catalytic agents employed, or the time of
heating are given

Answer 2

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here is your answer

$\boxed{\sf\pink{Kjeldahl's\:method}}$

✔ In this method, the compound

heated with concentrated sulphuric acid in the presence of small amount of ${CuSo_4}$to convert nitrogen into Ammonium Sulphate

✔The contents of the flask are transferred to another flask and heated with excess of sodium hydroxide solution to liberate ammonia gas

✔ Ammonia gas so liberated is passed and absorbed known as the volume concentrated sulphuric acid

✔Organic compound +${H_2SO_4{\rightarrow}(NH_4)_2SO_4}$

${(NH_4)_2SO4+2NaOH{\rightarrow}Na_2SO_4+2H_2O+72NH_3}$

✔${2NH_3+H_2SO_4{\rightarrow}(NH_4)_2SO_4}$